Crosslinked and crosslinkable hollow fiber membrane and method of making same utility

ABSTRACT

A composition of and a method of making high performance hollow fiber membranes is described. The membranes have a high resistance to plasticization by use of a predetermined amount of crosslinking. The preferred polymer material for the membrane is a polyimide polymer comprising covalently bonded ester crosslinks. The resultant hollow fiber membrane exhibits a high permeability of CO 2  in combination with a high CO 2 /CH 4  selectivity. Another embodiment provides a method of making the hollow fiber membrane from a monesterified polymer followed by final crosslinking after hollow fiber formation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 10/323,091 filed onDec. 18, 2002 which is a continuation in part of U.S. Ser. No.10/032,255 filed on Dec. 20, 2001 (now abandoned) both of which areherein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the separation of mixtures usingpolymer membranes.

BACKGROUND

Polymer membranes have been proposed for various separations. It hasbeen found that different molecules can be made to diffuse throughselected polymers differently. For example if one component of a mixtureis found to diffuse though a polymer rapidly and a second component isfound to diffuse through the polymer very slowly or not at all, thepolymer may be utilized to separate the two components. Polymermembranes potentially can be used for gas separations as well as liquidseparations.

Polymeric membrane materials have been found to be of use in gasseparations. Numerous research articles and patents describe polymericmembrane materials (e.g., polyimides, polysulfones, polycarbonates,polyethers, polyamides, polyarylates, polypyrrolones, etc.) withdesirable gas separation properties, particularly for use inoxygen/nitrogen separation (See, for example, Koros et al., J. MembraneSci., 83, 1-80 (1993), the contents of which are hereby incorporated byreference, for background and review).

The polymeric membrane materials are typically used in processes inwhich a feed gas mixture contacts the upstream side of the membrane,resulting in a permeate mixture on the downstream side of the membranewith a greater mole fraction of one of the components than thecomposition of the original feed gas mixture. A pressure differential ismaintained between the upstream and downstream sides, providing thedriving force for permeation. The downstream side can be maintained as avacuum, or at any pressure below the upstream pressure.

The membrane performance is characterized by the flux of a gas componentacross the membrane. This flux can be expressed as a quantity called thepermeability (P), which is a pressure- and thickness-normalized flux ofa given component. The separation of a gas mixture is achieved by amembrane material that permits a faster permeation rate for onecomponent (i.e., higher permeability) over that of another component.The efficiency of the membrane in enriching a component over anothercomponent in the permeate stream can be expressed as a quantity calledselectivity. Selectivity can be defined as the ratio of thepermeabilities of the gas components across the membrane (i.e.,P_(A)/P_(B), where A and B are the two components). A membrane'spermeability and selectivity are material properties of the membranematerial itself, and thus these properties are ideally constant withfeed pressure, flow rate and other process conditions. However,permeability and selectivity are both temperature-dependent. It isdesired to develop membrane materials with a high selectivity(efficiency) for the desired component, while maintaining a highpermeability (productivity) for the desired component.

The relative ability of a membrane to achieve the desired separation isreferred to as the separation factor or selectivity for the givenmixture. There are however several other obstacles to use of aparticular polymer to achieve a particular separation under any sort oflarge scale or commercial conditions. One such obstacle is permeationrate. One of the components to be separated must have a sufficientlyhigh permeation rate at the preferred conditions or else extraordinarilylarge membrane surface areas are required to allow separation of largeamounts of material. Another problem that can occur is that atconditions where the permeability is sufficient, such as at elevatedtemperatures or pressures, the selectivity for the desired separationcan be lost or reduced. Another problem that often occurs is that overtime the permeation rate and/or selectivity is reduced to unacceptablelevels. One problem that can occur is that one or more components of themixture can alter the form or structure of the polymer membrane overtime thus changing its permeability and/or selectivity. One specific waythis can happen is if one or more components of the mixture causesplasticization of the polymer membrane. Plasticization occurs when oneor more of the components of the mixture causes the polymer to swell andlose its membrane properties. It has been found that polymers such aspolyimides which have particularly good separation factors forseparation of mixtures comprising carbon dioxide and methane are proneto swelling and plasticization over time thus resulting in decreasingperformance of the membranes made from the polyimides.

The present invention overcomes some of the problems of the prior artmembranes by providing a polymer membrane and a route to making saidpolymer membrane that has the following properties/advantages:

-   -   a) Excellent selectivity and permeability,    -   b) Sustained selectivity over time by resistance to        plasticization, and    -   c) Very large useable surface area by use of hollow fibers.

SUMMARY

As discussed above the present invention seeks to provide a membrane andmethod of making the membrane that achieves the result of providing acommercially viable polymer membrane that overcomes some of thedrawbacks of the prior art membranes. The membranes of the presentinvention can have very large available surface areas using hollow fibertechnology. The membranes of the present invention also have a very highselectivity at a very high permeability. The membranes of the presentinvention also are quite resistant to plasticization and maintain theirselectivity and permeability properties over time as is required incommercial applications of this technology. The membrane of the presentinvention achieves this result by providing a predetermined number ofcrosslinkable sites in the polymer chain and by crosslinking the polymermembrane using selected crosslinking agents.

In one embodiment of the present invention a hollow fiber polymermembrane is provided, comprising; a crosslinked polyimide polymer havingcovalent ester crosslinks; and having a CO₂ permeance of at least 20 GPUand a CO₂/CH₄ selectivity of greater than 20, when measured at 35degrees C. and a pressure of 100 psia.

The productivity (permeance) of a gas separation membrane is measured inGPUs which is defined as follows:${GPU} = \frac{10^{- 6} \times {{cm}^{3}({STP})}}{{cm}^{2} \times {\sec.} \times \left( {{cm}.\quad{Hg}} \right)}$

In an alternative embodiment of the present invention a hollow fiberpolymer membrane is provided, comprising: a crosslinked polyimidepolymer having at least some covalent ester crosslinks and having aratio of crosslinkable sites to imide groups of between 3:8 and 1:16. Ithas been found that too much crosslinking can cause the hollow fiberpolymer to be fragile and can also result in poor membrane performance.Too little crosslinking can lead to plasticization of the polymermembrane over time resulting in deteriorating performance and loss ofselectivity.

In another alternative embodiment of the present invention a hollowfiber polymer membrane is described, comprising: a polyimide polymermade from the monomers A+B+C;

-   -   where A is a dianhydride of the formula;    -   where X₁ and X₂ are the same or different halogenated alkyl        group, phenyl or halogen;    -   where R₁, R₂, R₃, R4, R₅, and R₆ are H, alkyl, or halogen;    -   where B is a diamino cyclic compound without a carboxylic acid        functionality;    -   where C is a diamino cyclic compound with a carboxylic acid        functionality; and        wherein the ratio of B to C is between 1:4 and 8:1, and wherein        said hollow fiber polymer membrane material further comprises at        least some covalent ester crosslinks.

A particularly preferred embodiment of the present invention relates tousing the crosslinked hollow fiber polymer membrane of the presentinvention for the separation of carbon dioxide (CO₂) from methane (CH₄).In particular this embodiment of the invention relates to the removal ofCO₂ from natural gas comprising CO₂, CH₄, and other gases.

Among other factors, the present invention provides the composition ofand the method of making a highly effective polymeric membrane for theseparation of mixtures. The invention utilizes crosslinking of thepolymer membrane to help achieve the high selectivity required to makethe separation efficiently and to maintain the high selectivities andother properties even after being exposed to extreme conditions such ashigh temperatures and pressures. The invention also shows thatplasticization of the polymer membrane can be avoided by appropriatedegrees of crosslinking and appropriate selection of the crosslinkingunits. It has also been determined that too much crosslinking can leadto hollow fibers that are brittle and subject to failure. Anotherfeature of the present invention is that the selection of polymer havinga proper molecular weight (MW) can be important in the formation of ahollow fiber membrane. It is preferable to have a MW above theentanglement molecular weight of the polymer. It has been found that ifthe molecular weight of the polymer is too low the polymer is toobrittle and a proper skin layer may not form. If the molecular weight istoo high processability can become difficult. It is preferable to havean average molecular weight of between 20,000 and 200,000. The presentinvention has thus achieved a hollow fiber polymer membrane that is bothhighly selective and highly permeable for the preferred separationswhile also being stable and durable for long term use in a commercialseparation process at practical working conditions. The presentinvention also provides a method of making a hollow fiber polymermembrane material that is not excessively fragile, thereby allowingeffective spinning.

A preferred method for preparing hollow fibers is to dissolve thepolymer in a solvent or melt the polymer, and extrude the polymerthrough an annular capillary nozzle with a core fluid used for thepurpose of retaining the hollow fiber geometry.

Any gases that differ in size and condensability, for example nitrogenand oxygen or carbon dioxide and methane, can be separated using themembranes described herein. In one embodiment, a gaseous mixturecontaining methane and carbon dioxide can be enriched in methane by agas-phase process through the membrane. In other embodiments, themembranes can be used to purify helium, hydrogen, hydrogen sulfide,oxygen and/or nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the Monoesterification and Transesterification Reactions.

FIG. 2 shows the synthesis of the monoester via the acid chloridecopolyimide route.

FIG. 3 is a proton NMR of an uncrosslinked polymer (pre-esterification).

FIG. 4 is a proton NMR of the same polymer as FIG. 3 that has beenmonoesterified with 1,4-butanediol.

FIG. 5 is a proton NMR of a polymer that has been monoesterified withethylene glycol.

FIG. 6 is a Single Fiber Test Module.

FIG. 7 is a permeation testing system for membrane fiber modules.

FIG. 8 shows the CO₂ permeability at increasing pressures for acrosslinked material.

FIG. 9 shows the CO₂ permeability at increasing pressures for anuncrosslinked material.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a highly durable hollow fiber membraneexhibiting both high permeability of CO₂ and high CH₄/CO₂ selectivityand being resistant to plasticization. Prior membranes have shown asignificant decline in selectivity over time. Not to be limited bytheory, it is believed that the selectivity losses associated withexposure to high levels of CO₂ or other plasticizing agents are theresult of plasticization. Carbon dioxide acts as a strong swellingagent, sorbing into the polymer matrix and greatly increasing segmentalmotion. This increased motion drastically reduces the difference indiffusion rates between fast and slow gas species. If this swelling andsegmental motion could be limited, the selectivity of the membrane canbe maintained. In the present invention crosslinking has been shown toreduce or eliminate CO₂ plasticization in dense films. Proper selectionof the method of crosslinking, the chemical structure of the polymer andcrosslinking agent, and proper degree of crosslinking are important tomanufacture a hollow fiber membrane that achieves and maintains thesuperior permeability and selectivity needed for a viable commercialmembrane.

The polymeric fiber used is any suitable polyimide spun by anyconventional method, e.g., spun from a polymer solution through aspinneret. The polyimide is derived from a reaction of any suitablereactants. Reactants can include monomers such as dianhydrides, as wellas tetra carboxylic acids, and furandiones. Other monomers includediamino compounds, preferably diamino cyclic compounds, still morepreferably diamino aromatics. The diamino aromatics can include aromaticcompounds having more than one aromatic ring where the amino groups areon the same or different aromatic ring. In the present invention it isalso important for the polyimide to have incorporated in it apredetermined amount of crosslinkable sites. These sites may include butare not limited to carboxylic acid sites, ester functions, —OH groups,unreacted NH₂ groups, —SH groups, amide functions, and olefins. Thepreferred crosslinkable sites in the process of the present inventionare carboxylic acid or ester groups, alcohols, and olefins. Crosslinkingcan also be induced by reaction of the imide function itself to form acrosslinkable site and an amide. Another preferred feature of theprocess of the present invention is that the polyimide chains havelimited rotational ability. One such monomer that provides a polyimidechain with limited rotational ability is:

This dianhydride is known as 6FDA or 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, or (2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride.

In the process of the present invention a carboxylic acid functionalityis intended to include the acid group itself as well as acid derivativessuch as esters and anhydrides as well as activated carboxylic acidderivatives such as acid chlorides.

A preferred monomer for providing the carboxylic acid functionality inthe present invention is diamino benzoic acid:

A particularly preferred monomer is 3,5 diaminobenzoic acid:

The diamino cyclic compounds without a carboxylic acid functionality caninclude aromatic compounds having more than one aromatic ring where theamino groups are on the same or different aromatic ring. Preferredexamples include but are not limited to 4,4′ isopropylidene dianiline,3,3′ hexafluoroisopropylidene dianiline, 4,4′ hexafluoroisopropylidenedianiline, 4,4′ oxydianiline, 3,3′ oxydianiline and 4,4′diaminodiphenyl. Examples of diamino aromatic compounds useful in thepresent invention include diaminotoluene, diaminobenzotrifluoride, anddi, tri, and tetramethyldiaminobenzene.

The polymer membranes of the present invention can be used for gas/gasseparations, gas/liquid separations, liquid/liquid separations, andliquid/solid separations.

As mentioned above one of the preferred crosslinkable sites comprisecarboxylic acid or esters or activated carboxylic acid derivatives.Crosslinking groups or agents that have been found to be useful inconjunction with the carboxylic acid functional sites include: diolsselected from the group consisting of ethylene glycol, propylene glycol(1,2 propanediol), 1,3 propanediol, 1,4 butanediol, 1,2 butanediol,benzenedimethanol, and 1,3 butanediol. Preferred crosslinking agentsinclude ethylene glycol, propylene glycol, 1,3 propanediol, andbenzenedimethanol. More preferred crosslinking agents are ethyleneglycol, propylene glycol and 1,3 propanediol. Still more preferredcrosslinking agents are ethylene glycol, and 1,3 propanediol. It hasbeen found that having too long a crosslinking group can have anundesirable impact on the permeability and/or selectivity of the polymerhowever too short a crosslinking group can also have a negative effecton the finished hollow fiber membrane. The most preferred crosslinkingagents for crosslinking carboxylic acid or ester sites is 1,3propanediol.

Crosslinking can occur by the condensation reaction of selected diolswith the crosslinkable acid functionality. In the process of the presentinvention it has been found that reaction of less reactive crosslinkingagents can be facilitated by activation of the carboxylic acid site onthe polymer chain. One way to do this is by converting the acid group tothe corresponding acid chloride. This can be effectively done by the useof thionyl chloride. A method for this activation will be discussed inmore detail in the examples.

In a preferred embodiment of the process of the present inventioncrosslinking can be achieved in a stepwise fashion by firstmonoesterification of the acid function with the selected diol or diols,followed by transesterification of the monoester to the diester. (SeeFIG. 1)

In a particularly preferred embodiment of the present invention themonoesterified polymer is spun into the hollow fiber prior totransesterification to form the crosslinked hollow fiber membrane. Thereare significant advantages to this process in particular the monoesterpolymer can be more easily spun without breaking or forming defects.

It has been found transesterification can be accomplished by heating ofthe monoesterified polymer. In a particularly preferred embodiment ofthe present invention, where the hollow fiber is spun prior to thetransesterification to form the final crosslinked hollow fiber, careneeds to be taken to avoid damage to the hollow fiber. Preferablyheating is at a temperature high enough to cause substantialcrosslinking but not so high as to cause deformation of the hollowfiber. Most preferably the hollow fiber is not heated above about 200degrees C.

Crosslinking can be facilitated by various means such as heating, UVtreatment, microwaves, catalytic etc.

Alcohol or —OH groups can also provide crosslinkable sites in thepresent invention. Crosslinking groups useable with alcoholcrosslinkable sites include dicarboxylic acids, anhydrides, anddiesters. Examples of dicarboxylic acids useful as crosslink groupsinclude but are not limited to oxalic acid, malonic acid, succinic acid,methylsuccinic acid, glutaric acid, and adipic acid. Non limitingexamples of anhydrides that may be used include maleic anhydride,succinic anhydride, and methylsuccinic anhydride. Non limiting examplesof diesters are dimethylterephthalate, dimethylisophthalate,dimethylphthalate, and diesters of the dicarboxylic acids mentionedabove. The dicarboxylic acids and anhydrides can be reacted with the —OHcontaining polyimide at esterification conditions to form a crosslink.Likewise the diesters discussed above can be subjected totransesterification conditions in the presence of the —OH containingpolyimide to form the desired ester crosslink.

In a preferred embodiment of the present invention the —OH containingpolyimide is subjected to monoesterification conditions in the presenceof one or more of the crosslinking groups to form a monoesterifiedpolyimide. It has been found that the monoesterified polyimide can thenbe made into a hollow fiber. The hollow fiber can then be subjected totransesterification conditions after hollow fiber formation to form thecrosslinked hollow fiber polymer membrane.

Examples of reactants that can be used to provide an —OH containingpolyimide include diaminobenzyl alcohol, diaminocyclohexanol, and otherdiaminoalcohols.

In some cases it may be preferable to protect the —OH function prior toformation of the polyimide. This may be done by conventional chemicalmeans such as by masking the —OH group as an ether. The masked —OH groupmay then be hydrolyzed back to a functional —OH group prior tocrosslinking or prior to the extrusion of the hollow fiber.

Also mentioned above are crosslinkable sites comprising olefins.Crosslinking groups useable with olefins include but are not limited tosulfur, and divinylbenzene. Sulfur as a crosslinking agent is thought toform a disulfide crosslink when reacted with an olefin.

A particularly preferred diamino group that can be used to make acrosslinkable polyimide polymer is diaminobenzoic acid. The mostpreferred isomer is 3,5 diaminobenzoic acid (DABA).

Polymer Selection

An appropriately selected polymer can be used which permits passage ofthe desired gases to be separated, for example carbon dioxide andmethane. Preferably, the polymer permits one or more of the desiredgases to permeate through the polymer at different diffusion rates thanother components, such that one of the individual gases, for examplecarbon dioxide, diffuses at a faster rate through the polymer. In apreferred embodiment, the rate at which carbon dioxide passes throughthe polymer is at least 10 times faster than the rate at which methanepasses through the polymer.

It is preferred that the membranes exhibit a carbon dioxide/methaneselectivity of at least about 5, more preferably at least about 10,still more preferably at least 20, and most preferably at least about30. Preferably, the polymer is a rigid, glassy polymer as opposed to arubbery polymer or a flexible glassy polymer. Glassy polymers aredifferentiated from rubbery polymers by the rate of segmental movementof polymer chains. Polymers in the glassy state do not have the rapidmolecular motion that permit rubbery polymers their liquid-like natureand their ability to adjust segmental configurations rapidly over largedistances (>0.5 nm). Glassy polymers exist in a non-equilibrium statewith entangled molecular chains with immobile molecular backbones infrozen conformations. The glass transition temperature (Tg) is thedividing point between the rubbery or glassy state. Above the Tg, thepolymer exists in the rubbery state; below the Tg, the polymer exists inthe glassy state. Generally, glassy polymers provide a selectiveenvironment for gas diffusion and are favored for gas separationapplications. Rigid, glassy polymers describe polymers with rigidpolymer chain backbones that have limited intramolecular rotationalmobility and are often characterized by having high glass transitiontemperatures (Tg>150° C.).

In rigid, glassy polymers, the diffusive selectivity tends to dominate,and glassy membranes tend to be selective in favor of small, low-boilingmolecules. The preferred membranes are made from rigid, glassy polymermaterials that will pass carbon dioxide preferentially over methane andother light hydrocarbons. Such polymers are well known in the art andare described, for example, in U.S. Pat. No. 4,230,463 to Monsanto andU.S. Pat. No. 3,567,632 to DuPont. Suitable membrane materials includepolyimides, polysulfones and cellulosic polymers among others.

Examples of suitable polymers useable as either the membrane material orthe porous support include substituted or unsubstituted polymers and maybe selected from polysulfones; poly(styrenes), includingstyrene-containing copolymers such as acrylonitrilestyrene copolymers,styrene-butadiene copolymers and styrene-vinylbenzylhalide copolymers;polycarbonates; cellulosic polymers, such as cellulose acetate-butyrate,cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose,etc.; polyamides and polyimides, including aryl polyamides and arylpolyimides; polyethers; polyetherimides; polyetherketones;polyethersulfones; poly(arylene oxides) such as poly(phenylene oxide)and poly(xylene oxide); poly(esteramide-diisocyanate); polyurethanes;polyesters (including polyarylates), such as polyethylene terephthalate,poly(alkyl methacrylates), poly(acrylates), poly(phenyleneterephthalate), etc.; polypyrrolones; polysulfides; polymers frommonomers having alpha-olefinic unsaturation other than mentioned abovesuch as poly (ethylene), poly(propylene), poly(butene-1), poly(4-methylpentene-1), polyvinyls, e.g., poly(vinyl chloride), poly(vinylfluoride), poly(vinylidene chloride), poly(vinylidene fluoride),poly(vinyl alcohol), poly(vinyl esters) such as poly(vinyl acetate) andpoly(vinyl propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones),poly(vinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes) such aspoly(vinyl formal) and poly(vinyl butyral), poly(vinyl amides),poly(vinyl amines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinylphosphates), and poly(vinyl sulfates); polyallyls;poly(benzobenzimidazole); polyhydrazides; polyoxadiazoles;polytriazoles; poly (benzimidazole); polycarbodiimides;polyphosphazines; etc., and interpolymers, including block interpolymerscontaining repeating units from the above such as terpolymers ofacrylonitrile-vinyl bromide-sodium salt of para-sulfophenylmethallylethers; and grafts and blends containing any of the foregoing. Typicalsubstituents providing substituted polymers include halogens such asfluorine, chlorine and bromine; hydroxyl groups; lower alkyl groups;lower alkoxy groups; monocyclic aryl; lower acyl groups and the like.

Preferred polymers useable in the hollow fiber membrane of the presentinvention include polyimides, poyletherimides, polyethersulfones andpolysulfones. More preferred polymers useable in the membrane materialof present invention include polyimides, poyletherimides, andpolysulfones made using analogs of 6FDA. Particularly preferredpolyimides useable in the present invention comprise polyimides orpolyetherimides made using 6FDA.

In a particularly preferred embodiment of the present invention thehollow fiber polymer membrane is a composite material comprising amembrane layer comprising an effective skin layer as well as a poroussupport. The porous support material can be the same or differentpolymer as the membrane. Ideally the porous support is an inexpensiveporous polymer. In a composite hollow fiber polymer membrane the poroussupport layer can be either the inside layer or the outside layer. Mostpreferably the porous support layer is the inside layer in thisembodiment and the “skin” layer is on the outside of the hollow fiber. Acomposite membrane material is discussed in copending U.S. patentapplication Ser. Nos. 09/834,857 and 09/834,808 which are incorporatedherein in their entirety. A Patent that discusses composite membranes isU.S. Pat. No. 4,925,459 which is also incorporated herein by referencein its entirety.

Molecular Weight of the Polymer

Another parameter that needs to be controlled in order to achieve thehigh permeability, high selectivity hollow fiber membrane of the presentinvention is the molecular weight of the polymer material. Molecularweight of the polymer material can be critical to forming a hollow fibermembrane that is not too brittle and has an effective skin layer.Molecular weight of the polymer material can also be critical inachieving a spinnable dope solution. A feature of the present inventionis that the selection of polymer having a proper molecular weight (MW)can be important in the formation of a hollow fiber membrane. It ispreferable to have a MW above the entanglement molecular weight of thepolymer. It has been found that if the molecular weight of the polymeris too low the polymer is too brittle and an effective skin layer maynot form. If the molecular weight is too high processability can becomedifficult. In the present invention it is preferable to have an averagepolymer molecular weight of between 20,000 and 200,000, more preferablybetween 30,000 and 160,000, still more preferably between 40,000 and140,000, and most preferably between 60,000 and 120,000. Not to belimited by theory, it is thought that the MW of the polymer should beabove, ideally well above, the entanglement MW of the polymer in orderto achieve a material that has high strength and is not brittle. A paperthat discusses the effect of molecular weight on polymer properties suchas entanglement is in Fundamental Principles of Polymeric Materials, SPEMonograph Series 2^(nd) ed., John Wiley & Sons, New York: (1982), page259 written by Stephen L. Rosen; the contents of which are herebyincorporated by reference, for background and review.

It is also believed that the molecular weight of the polyimide chain canbe degraded during the monoesterification process. A sufficiently highmolecular weight polymer should be used to allow for some loss of MWduring the esterification process yet still be within the desired rangeafter completion. The molecular weights used in the present applicationare Weight Average Molecular Weights and can be determined by GPC (GelPermeation Chromatography).

Separation Systems Including the Membranes

The membranes may take any form known in the art, for example hollowfibers, tubular shapes, and other membrane shapes. Some other membraneshapes include spiral wound, pleated, flat sheet, or polygonal tubes.Multiple hollow fiber membrane tubes can be preferred for theirrelatively large fluid contact area. The contact area may be furtherincreased by adding additional tubes or tube contours. Contact may alsobe increased by altering the gaseous flow by increasing fluid turbulenceor swirling.

The preferred glassy materials that provide good gas selectivity, forexample carbon dioxide/methane selectivity, tend to have relatively lowpermeabilities. A preferred form for the membranes is, therefore,integrally skinned or composite asymmetric hollow fibers, which canprovide both a very thin selective skin layer and a high packingdensity, to facilitate use of large membrane areas.

Hollow fibers can be employed in bundled arrays potted at either end toform tube sheets and fitted into a pressure vessel thereby isolating theinsides of the tubes from the outsides of the tubes. Devices of thistype are known in the art. Preferably, the direction of flow in a hollowfiber element will be counter-current rather than co-current or eventransverse.

Sheets can be used to fabricate a flat stack permeator that includes amultitude of membrane layers alternately separated by feed-retentatespacers and permeate spacers. The layers can be glued along their edgesto define separate feed-retentate zones and permeate zones. Devices ofthis type are described in U.S. Pat. No. 5,104,532, the contents ofwhich are hereby incorporated by reference.

The membranes can be included in a separation system that includes anouter perforated shell surrounding one or more inner tubes that containthe membranes. The shell and the inner tubes can be surrounded withpacking to isolate a contaminant collection zone.

In one mode of operation, a gaseous mixture enters the separation systemvia a containment collection zone through the perforations in the outerperforated shell. The gaseous mixture passes upward through the innertubes. As the gaseous mixture passes through the inner tubes, one ormore components of the mixture permeate out of the inner tubes throughthe selective membrane and enter the containment collection zone.

The membranes can be included in a cartridge and used for permeatingcontaminants from a gaseous mixture. The contaminants can permeate outthrough the membrane, while the desired components continue out the topof the membrane. The membranes may be stacked within a perforated tubeto form the inner tubes or may be interconnected to form aself-supporting tube.

Each one of the stacked membrane elements may be designed to permeateone or more components of the gaseous mixture. For example, one membranemay be designed for removing carbon dioxide, a second for removinghydrogen sulfide, and a third for removing nitrogen. The membranes maybe stacked in different arrangements to remove various components fromthe gaseous mixture in different orders.

Different components may be removed into a single contaminant collectionzone and disposed of together, or they may be removed into differentzones. The membranes may be arranged in series or parallelconfigurations or in combinations thereof depending on the particularapplication.

The membranes may be removable and replaceable by conventional retrievaltechnology such as wire line, coil tubing, or pumping. In addition toreplacement, the membrane elements may be cleaned in place by pumpinggas, liquid, detergent, or other material past the membrane to removematerials accumulated on the membrane surface.

A gas separation system including the membranes described herein may beof a variable length depending on the particular application.

The gaseous mixture can flow through the membrane(s) following aninside-out flow path where the mixture flows into the inside of thetube(s) of the membranes and the components which are removed permeateout through the tube. Alternatively, the gaseous mixture can flowthrough the membrane following an outside-in flow path.

In order to prevent or reduce possibly damaging contact between liquidor particulate contaminates and the membranes, the flowing gaseousmixture may be caused to rotate or swirl within an outer tube. Thisrotation may be achieved in any known manner, for example using one ormore spiral deflectors. A vent may also be provided for removing and/orsampling components removed from the gaseous mixture.

The membranes are preferably durable, resistant to high temperatures,and resistant to exposure to liquids. The materials may be coated,ideally with a polymer, to help prevent fouling and improve durability.Examples of suitable polymers include those described in U.S. Pat. Nos.5,288,304 and 4,728,345, the contents of which are hereby incorporatedby reference. Barrier materials may also be used as a pre-filter forremoving particulates and other contaminants which may damage themembranes.

Methods of Forming Hollow Fibers

Hollow fibers can be formed, for example, by extruding a polymersolution through an annular capillary nozzle with a core fluid used forthe purpose of retaining the hollow fiber geometry. These fiberstypically have a diameter similar to a human hair and offer theadvantage of very high surface area per unit volume. Industrial hollowfiber membrane modules typically contain hundreds of thousands ofindividual hollow fibers. Specifically, to maximize productivity, thehollow fibers typically include an ultrathin (<2000 Angstroms) “skin”layer on a porous support. Gas separation is accomplished through thisselective “skin.” This outer “skin” layer may be supported on the samepolymer to form an integrally skinned asymmetric hollow fiber membrane.The most advanced membranes have an asymmetric sheath with the selectiveskin supported on an inexpensive porous core layer (different polymer)to form a composite hollow fiber membrane. This type of device isdescribed in U.S. Pat. No. 5,085,676, the contents of which are herebyincorporated by reference.

Hollow fibers can be employed in bundled arrays potted at either end toform tube sheets and fitted into a pressure vessel thereby isolating theinsides of the tubes from the outsides of the tubes. Devices of thistype are known in the art. Preferably, the direction of flow in a hollowfiber element will be counter-current rather than co-current or eventransverse. Such counter-current flow can be achieved by wrapping thehollow fiber bundle in a spiral wrap of flow-impeding material. Thisspiral wrap extends from a central mandrel at the center of the bundleand spirals outward to the outer periphery of the bundle. The spiralwrap contains holes along the top and bottom ends whereby gas enteringthe bundle for tube side flow at one end is partitioned by passagethrough the holes and forced to flow parallel to the hollow fiber downthe channel created by the spiral wrap. This flow direction iscounter-current to the direction of flow inside the hollow fiber. At thebottom of the channels the gas re-emerges from the hollow fiber bundlethrough the holes at the opposite end of the spiral wrap and is directedout of the module.

A viscosity enhancing agent or viscosity enhancing salt may be usefulfor making a spinning solution (dope) suitable for spinning. Viscosityenhancing salts can be most useful when the molecular weight of thepolymer is near the low end of the MW range discussed elsewhere in thisapplication. One possible viscosity enhancing salt useable in thepresent invention is lithium nitrate (LiNO₃). The use of a viscosityenhancing salt is taught is Example 7 of the present application. Use ofviscosity enhancers and other spinning conditions are also taught inPolyaramide hollow fibers for H ₂ /CH ₄ separation II. Spinning andProperties by Ekiner and Vassilatos Journal of Membrane Science 186(2001) 71-84 which is hereby incorporated by reference in its entirety.

The standard unit for measuring the permeability of gases through asupported gas separation membrane is the Barrer, which is defined asfollows:${1\quad{Barrer}} = \frac{10^{- 10}\quad{{cm}^{3}({STP})} \times {cm}}{{cm}^{2} \times {\sec.} \times \left( {{cm}.\quad{Hg}} \right)}$

-   -   wherein the flux (flow rate) in units of cm3/cm² .X sec.; being        volume per seconds of permeated gas at standard temperature and        pressure,

-   cm is the thickness of the film,

-   cm² is the area of film, and

-   cm. Hg is the pressure (or driving force).

The selectivity of a supported gas separation membrane in separating atwo-component fluid mixture is defined as the ratio of the rate ofpassage of the more readily passed component to the rate of passage ofthe less readily passed component. Selectivity may be obtained directlyby contacting a supported gas separation membrane with a known mixtureof gases and analyzing the permeate. Alternatively, a firstapproximation of the selectivity is obtained by calculating the ratio ofthe rates of passage of the two components determined separately on thesame gas separation membrane. Rates of passage may be expressed inBarrer units. As an example of selectivity, a O₂/N₂=10 indicates thatthe subject membrane allows oxygen gas to pass through at a rate tentimes that of nitrogen.

The productivity (permeance) of a gas separation membrane is measured inGPUs which is defined as follows:${GPU} = \frac{10^{- 6} \times {{cm}^{3}({STP})}}{{cm}^{2} \times {\sec.} \times \left( {{cm}.\quad{Hg}} \right)}$Purification Process

A mixture containing gases to be separated, for example carbon dioxideand methane, can be enriched by a gas-phase process through themembrane, for example, in any of the above-configurations. The preferredconditions for enriching the mixture involve using a temperature betweenabout 25° C. and 200° C. and a pressure of between about 50 psia and5000 psia. These conditions can be varied using routine experimentationdepending on the feed streams. Other gas mixtures can be purified withthe membrane in any of the above configurations. For example,applications include enrichment of air by nitrogen or oxygen, nitrogenor hydrogen removal from methane streams, or carbon monoxide from syngasstreams. The membrane can also be used in hydrogen separation fromrefinery streams and other process streams, for example from thedehydrogenation reaction effluent in the catalytic dehydrogenation ofparaffins. Generally, the membrane may be used in any separation processwith gas mixtures involving, for example, hydrogen, nitrogen, methane,carbon dioxide, carbon monoxide, helium, and oxygen.

Additional Purification

If additional purification is required, the product in the permeatestream can be passed through additional membranes, and/or the productcan be purified via distillation using techniques well known to those ofskill in the art. Typically, membrane systems may consist of manymodules connected in various configurations (See, for example, Prasad etal., J. Membrane Sci., 94, 225-248 (1994), the contents of which arehereby incorporated by reference for background and review). Modulesconnected in series offer many design possibilities to purify the feed,permeate, and residue streams to increase the separation purity of thestreams and to optimize the membrane system performance.

As discussed above the membrane to be commercially viable must have highpermeability of at least one component in combination with excellentselectivity. Preferably the crosslinked polyimide polymer hollow fibermembrane of the present invention has a CO₂ permeance of at least 15 GPUand a CO₂/CH₄ selectivity of greater than 15, preferably the CO₂permeance is at least 20 GPU and the CO₂/CH₄ selectivity is greater than20, still more preferably the CO₂ permeance is greater than 25 and theCO₂/CH₄ selectivity is greater than 25, most preferably the CO₂permeance is greater than 25 and the CO₂/CH₄ selectivity is greater than30. The permeability and selectivity of the membrane is measured at 35degrees C. and a pressure of 100 psia.

Methodology of Fiber Module Construction

For laboratory or commercial use, a suitable plurality of the fibers isbundled together to form a separation unit. The number of fibers bundledtogether will depend on fiber diameters, lengths, and porosities and ondesired throughput, equipment costs, and other engineeringconsiderations understood by those in the chemical engineering arts.

The fibers are held together by any conventional means. This assembly isthen typically disposed in a pressure shell such that one end of thefiber assembly extends to one end of the pressure shell and the oppositeend of the fiber assembly extends to the opposite end of the pressureshell. The fiber assembly is then fixably or removably affixed to thepressure shell by any conventional method to form a pressure tight seal.

The unit is then operated, e.g., as a shell-tube heat exchanger, wherethe feed is passed to either the shell or tube side at one end of theassembly and the product is removed from the other end. For maximizinghigh-pressure performance, the high-pressure feed is typically fed tothe shell side of the assembly. At least a portion of the CO₂ in thefeed passes through the membrane to the tube side, i.e., inside themembranes. CO₂ depleted feed is then removed from the opposite end ofthe shell side of the assembly. Any conventional recycle scheme may beoptionally used to optimize a desired purity level.

In order to perform permeation tests, for example, a test moduleconsisting of a single fiber is constructed, as shown in FIG. 6. Detailsof fabricating the module are given in the Illustrated Embodimentssection below.

Operating Conditions

The process is operated with a feed pressure of from about 20 psia toabout 4000 psia, preferably at least about 50 psia, and more preferablyfrom about 200 psia to about 1000 psia. The feed temperature is itsambient temperature, e.g., its temperature as produced from the well.

Methodology of Single Fiber Module Construction

Reference is made to FIG. 6. In order to perform permeation tests, amodule 200 consisting of a single fiber 205 was constructed. The module200 is fabricated from two stainless steel (316) Swagelok® ¼-inch tees210, stainless steel ¼-inch tubing and nuts, two brass NPT ¼-inchfemale-tube adapters 215, two brass NPT ¼-inch male-tube adapters 220,and two brass Swagelok® ¼-inch nuts. The hollow fiber membrane 205 isthreaded through the module housing, so that a length of carbon fiberextends on each end. The ends of the module are then plugged withStycast® 2651 epoxy 225 (from Emerson-Cuming Company) cured forovernight. The ends of the membrane 205 are snapped off after the epoxyhardens.

Methodology of Membrane Testing System

Reference is made to FIGS. 6 and 7. The permeation testing for thefibers 205 was performed with single-fiber test modules 200. Gastransport through the membranes was examined with a pressure-risepermeation testing system 300. The system permitted high-pressuretesting of mixed feed gas and sampling of gas streams with a gaschromatograph. The module 200 was attached in a shell feed method ofoperation. Mixed feed gas 305 from a compressed gas cylinder 310 wassupplied on the shell-side of a single-fiber test module 200. The module200 and ballast volumes were placed in a circulating water bath 315 tocontrol and maintain a constant temperature.

Vacuum was pulled on both the shell- and bore-side of the hollow fibermembrane 205 first for overnight before testing. Permeate at the twoends from the bore-side of the fiber was pulled by vacuum through adownstream sample volume. The permeation rate was measured from thepressure rise of a Baratron® pressure transducer 320 over time afterclosing the valve to vacuum. The pressure rise was plotted on chartrecorder. The compositions of all the streams can be determined by a gaschromatograph. Individual gas fluxes were then calculated. The plumbingof the system consisted of stainless steel (316) Swagelok® ¼-inch and⅛-inch fittings and tubing, Whitey® and Nupro® valves with weldedelements. The system is rated for over 1500 psia pressure.

EXAMPLES

The present invention will be better understood with reference to thefollowing non-limiting examples. The present examples are intended tohelp illustrate the process of the present invention and are not meantto limit the scope of the application.

Example 1 Synthesis of Monoester via Activated Carboxylic Acid

The reactivity of the diols strongly depends on their structure. Due tothe electron releasing effect of the methylene groups the reactivity ofdiols increases with increasing chain length. For example, 1,4butanediol>1,3-propane diol>ethylene glycol.

The monoesterification reaction was carried out as follows: theDABA-copolyimide is dissolved in THF (10 wt %) under nitrogen atmosphereand 2 times of the stoichiometric amount of thionyl chloride is added.The reaction is heated to reflux and the excess of thionyl chloride andTHF is distilled out of the reaction solution.

The residual copolyimide acid chloride was stored under vacuum at lowtemperature (50° C.) overnight. The acid chloride was dissolved in THFand dropped slowly to an excess of glycol (70 times excess) dissolved inTHF.

Example 2 Self Catalyzed Monoesterification Reaction

Some of the diols such as 1,4 butanediol have been found to form themonoester without the use of a catalyst. For the self-catalyzedreaction, DABA-copolyimides are dissolved in dry NMP (15-17 wt %) and 70times excess of diol is added. The reaction mixture is stirred for 12hours at 130° C. under nitrogen purge. Precipitation, blending andfiltration lead to fluffy particles of monoester which are dried at 70°C. under vacuum.

Example 3 Acid Catalyzed Monoesterification Reaction

For the acid catalysed reaction, per 2 g of polymer 1 mg of p-toluenesulfonic acid was added. The procedure for the reaction was the same asfor the self-catalyzed reaction.

Example 4 Conversion of the Monoesterification Reaction

We have found that ¹H-NMR is a useful method to show the conversionwhich can be reached in the monoesterification reaction. This should beexplained on two examples. FIG. 3 shows the ¹H NMR of 6FDA-DAM/DABA 3:2non-crosslinked in DMSO-D6. The presence of the DABA units can be provenby comparing the ratio of all aromatic protons and aliphatic protons (3methyl groups of DAM). After the self-catalyzed monoesterificationreaction with 1,4-butanediol and low temperature drying of the monoester(70° C. under vacuum), again ¹H NMR was performed. The spectrum is shownin FIG. 4. For the monoester-NMR we can calculate the conversions of thereaction by the ratio of aromatic protons and aliphatic protons of themethylene group next to the ester group. We can check the calculationsalso by the ratio of DAM-methyl protons and the methylene group next tothe ester group.

From the spectrum obtained after the monoesterification reaction it canbe concluded that nearly complete conversions can be obtained with1,4-butanediol using the self-catalyzed reaction conditions.

As already mentioned we assume that the ethylene glycol is lessnucleophilic than the butanediol. It has been found that using ethyleneglycol in a self-catalyzed monoesterification reaction, the conversionsseems to be much lower. The ¹H-NMR of the 6FDA-DAM/DABA 2:1 ethyleneglycol monoester is shown in FIG. 5.

Table 1 summarizes the results for the conversions obtained withcopolyimides having different DAM/DABA compositions. Thereby differentmethods for synthesizing the monoester copolyimide were investigated.The conversion of the reactions was independently calculated from theratio of aromatic protons (without the aromatic DAM proton) and themethylene protons next to the ester group as well as from the ratio ofthe aliphatic DAM methyl protons and the methylene protons next to theester. The following conclusions can be drawn:

The monoesterification reaction can be catalyzed by protons. Thereforeit is obvious that with increasing DABA content of the copolyimidestructure higher conversion rates are obtained. The DAM/DABA 4:1 withbutanediol shows a conversion of less than 50% whereas for the DAM/DABA3:2 a conversion of over 90% was obtained (self catalyzed). Ethyleneglycol generally has a lower nucleophilic character than butanediol, theconversion for a DAM/DABA 2:1 composition is rather low (less than 40%)although a high number of protons are present due to the high DABAcontent.

For DAM/DABA 4:1 monoesterification with ethylene glycol very lowconversion was expected (at least less than 40%) in the self-catalyzedreaction due to the low DABA content. By adding p-Toluene sulfonic acidto the monoesterification reaction conversions of more than 80% can bereached.

The acid chloride groups are highly reactive groups, thereforeconversions are over 95% for the monoester synthesis over the acidchloride route. TABLE 1 Calculated Conversion Calculated based onConversion Aromatics based on DAM- Ratio: (without DAM/ CH₃ protons/Aromatics (total)/ methylene methylene Copolyimide DAM-CH₃ protonsprotons (on Monoester protons (on ester) ester) DAM/DABA 4:1theoretical: 22 23 Butanediol 1.03 Self-catalyzed experimental: 1.08DAM/DABA 2:1 theoretical: 38 40 Ethylene 1.28 glycol Self- experimental:catalyzed 1.23 DAM/DABA 3:2 theoretical: 94 98 Butanediol 1.44Self-catalyzed experimental: 1.50 DAM/DABA 3:2 theoretical: 97 98Butanediol 1.44 Over acid experimental: chloride 1.49

Example 5 Plasticization Resistance

In order to show CO₂ plasticization resistance, pure CO₂ permeationexperiments have been performed with the 6FDA-DAM/DABA 3:2 filmcrosslinked with 1,4 butanediol at 140° C. To determine the CO₂plasticization the CO₂ permeability is measured at increasing CO₂pressure. The CO₂ pressure was held at a given pressure for 24 hoursthen the CO₂ permeability was measured. The CO₂ pressure was then heldfor an additional 24 hours and again measured. A substantial increase inthe CO₂ permeability indicates plasticization. Results of plasticizationtest are shown in FIG. 8. FIG. 8 shows that the crosslinked material hassurprising resistance to plasticization.

Comparative Example 6 Plasticization Resistance of Uncrosslinked Film

To contrast the plasticization resistance of a crosslinked membrane (ofExample 5), an uncrosslinked film was tested under similar conditions toExample 5. The results of this test are shown in FIG. 9. FIG. 9 shows asubstantial change in the CO₂ permeability indicating plasticization.

Example 7 Spinning of Crosslinked Defect Free Asymmetric Hollow Fiber

A spinning solution (dope) containing polyimide, N-methyl-2-pyrilodinone(NMP), ethanol, and a viscosity enhancing salt (LiNO₃) was mixed to forma homogenous solution. The polyimide used was made from the 6FDAdianhydride (4,4′-[Hexafluoroisopropylidene]diphthalic anhydride) and a3:2 ratio of DAM (2,4,6-trimethyl-1,3-phenylene diamine) to DABA(diamino benzoic acid) diamines. Over 98% of the DABA groups had beenreacted with propane diol to form the monoester form of the polymer. Thedope was rolled in a sealed container for 5 days to ensure completemixing. The dope was then allowed to degas for 24 hours before beingpoured into an ISCO® syringe pump, where it was again degassed for 24hours.

The dope was extruded from an annular spinneret at 0.8 mL/min through anair gap into a quench bath filled with deionized water and taken up on arotating drum at between 14 and 16 m/min. A solution consisting of 90%NMP with 10% water was used as the bore fluid. The fibers were keptwetted with DI water while on the take-up drum. The fibers were cut fromthe drum with a razor to lengths of one meter and washed in DI water for24 hours.

After washing in water, the fibers were washed in baths of ethanol (2×30min) and hexane (2×30 min). The hexane-wet fibers were allowed to airdry for 30 minutes and then dried under vacuum at 120° C. for one hour.

The fibers were crosslinked by exposure to 150° C. for 25 hours undervacuum. They were subsequently potted into modules and tested forpermeation properties.

Example 8 Testing of Crosslinked Hollow Fibers

Crosslinked fibers (see Example 7) were potted into modules, eachcontaining 5-10 fibers with an active length of approximately 20 cm.Pure gases were fed on the shell side at 50 psig and the flux throughthe fibers was measured with bubble flow meters. Atmospheric pressurewas maintained on the downstream side and the overall temperature wasnear 25° C. The flux measured with the bubble flow meters was convertedto permeance and the results are shown in the table below. N₂ permeance(GPU) 1.7 O₂ permeance (GPU) 6.5 He permeance (GPU) 52 CH₄ permeance(GPU) 1.1 CO₂ permeance (GPU) 23 O₂/N₂ selectivity 3.8 He/N₂ selectivity31 CO₂/CH₄ selectivity 21

High pressure nitrogen was used to test the crush pressure of thefibers. Using the same setup described above, nitrogen was fed on theshell side, beginning at 50 psig. The pressure was increased every 30minutes in 50 psig increments and the permeance was measured. A drasticchange in permeance after increasing the pressure is indicative of fibercollapse. The fibers maintained their structural integrity up to 900psig of pure nitrogen.

Plasticization resistance of the fibers was tested using a similarprocedure to that used in crush testing. In this case, the test gas wasCO₂, pressure was increased every 60 minutes, and permeance measurementswere taken every 30 minutes (after 30 and 60 minutes of exposure to agiven pressure). Plasticization was indicated by a sharp increase in theslope of the permeance vs. pressure curve. For the crosslinked fibers,this occurred at about 250 psig of pure CO₂, as compared to less than 50psig for uncrosslinked fibers of the same material.

1. A composite polymer membrane, comprising: a crosslinked polyimidepolymer having covalent ester crosslinks; and having a CO₂ permeance ofat least 20 GPU and a CO₂/CH₄ selectivity of greater than 20, at 35degrees C. and a pressure of 100 psia.
 2. A composite polymer membrane,comprising: a crosslinked polyimide polymer having at least somecovalent ester crosslinks and having a ratio of crosslinkable sites toimide groups of between 3:8 and 1:16.
 3. A composite polymer membrane,comprising: a polyimide polymer made from the monomers A+B+C; where A isa dianhydride of the formula;

where X₁ and X₂ are the same or different halogenated alkyl group,phenyl or halogen; where R₁, R₂, R₃, R₄, R₅, and R₆ are H, alkyl, orhalogen; where B is a diamino cyclic compound without a carboxylic acidfunctionality; where C is a diamino cyclic compound with a carboxylicacid functionality; and wherein the ratio of B to C is between 1:4 and8:1, and wherein said composite polymer membrane further comprises atleast some covalent ester crosslinks.
 4. The composite polymer membraneof claim 3 where X₁ and X₂ are CF₃.
 5. The composite polymer membrane ofclaim 3 where R₁, R₂, R₃, R₄, R₅, and R₆ are H.
 6. The composite polymermembrane of claim 3 wherein the dianhydride is 6FDA.
 7. The compositepolymer membrane of claim 3 wherein C is DABA.
 8. The composite polymermembrane of claim 3 wherein B is a diamino aromatic compound.
 9. Thecomposite polymer membrane of claim 3 wherein B is a methyl substituteddiamino benzene.
 10. The composite polymer membrane of claim 3 whereinthe ratio of B to C is between 17:3 and 3:2.
 11. The composite polymermembrane of claim 3 wherein said ester crosslinks are made using a diolselected from the group consisting of ethylene glycol, propylene glycol,1,3 propanediol, 1,4 butanediol, 1,2 butanediol, benzenedimethanol, and1,3 butanediol.
 12. The composite polymer membrane of claim 1 whereinthe membrane is a flat sheet.
 13. The composite polymer membrane ofclaim 13 wherein the membrane comprises a layer in a flat stackpermeator.
 14. The composite polymer membrane of claim 1 wherein theester crosslinks are formed by the monoesterification of a polyimidepolymer having carboxylic acid sites incorporated therein, with a diol.15. The composite polymer membrane of claim 16 wherein the diol isselected from the group consisting of ethylene glycol, propylene glycol,1,3 propanediol, 1,4 butanediol, 1,2 butanediol, benzenedimethanol, and1,3 butanediol.
 16. The composite polymer membrane of claim 3 whereinthe ratio of B to C is between 4:1 and 3:2.
 17. The composite polymermembrane of claim 3 wherein the polyimide polymer has an averagemolecular weight of between 20,000 and 200,000.
 18. The compositepolymer membrane of claim 3 wherein the membrane has an averagemolecular weight greater than the entanglement MW of the polymer. 19.The composite polymer membrane of claim 3 wherein the membrane has anaverage molecular weight of between 40,000 and 140,000.